1. Field of the Invention
The present invention generally relates to an optical attenuator apparatus and, more particularly, to an optical attenuator, used in optical communication, for attenuating light incident from a light source by a desired amount and outputting attenuated light so as to prevent optical level variation during attenuation amount switching.
2. Description of the Related Art
FIG. 28 shows a general schematic arrangement of an optical attenuator 40 used in optical communication.
Schematically, the optical attenuator 40 has incident- and exit-side connectors 42 and 43 provided to two sides of an attenuator body 41, two sets of lenses 45 and 46 for collimating light supplied from a light source 44 to the incident-side connector 42 and guiding parallel light to the exit-side connector 43, and two attenuation filters 49 and 50, arranged on the optical path between the lenses 45 and 46, for attenuating light by a desired amount and outputting the attenuated light to the exit-side connector 43.
As the attenuation filters 49 and 50, those shown in FIGS. 29 and 30 are conventionally generally used.
More particularly, attenuation filters 49 and 50 shown in FIG. 29 have attenuation plates 49a and 50a. In the attenuation plate 49a, a metal film whose transmittance changes continuously in the circumferential direction is formed on a disc-shaped glass substrate in order to obtain continuous attenuation amounts (e.g., 0 to 10 dB). In the attenuation plate 50a, filters 50aa having predetermined different attenuation amounts are provided at several portions of a disc-shaped metal substrate in order to obtain a plurality of fixed attenuation amounts (e.g., 10, 20, . . . , 50 dB). The attenuation plates 49a and 50a are arranged to partially overlap each other on an optical path along which light is transmitted.
Attenuation filters 49 and 50 shown in FIG. 30 have attenuation plates 49b and 50b. In the attenuation plate 49b, a metal film whose transmittance changes continuously along the circumferential direction is formed on a disc-shaped glass substrate to obtain continuous attenuation amounts (e.g., 0 to 10 dB). In the attenuation plate 50b, a plurality of filters 50bb having predetermined fixed attenuation amounts are provided on a disc-shaped glass substrate at predetermined angular intervals to obtain a plurality of fixed attenuation amounts (e.g., 10, 20, . . . , 50 dB). The attenuation plates 49b and 50b are arranged to partially overlap each other on an optical path along which light is transmitted.
In FIG. 28, to attenuate light, supplied from the light source 44 and incident through the incident-side connector 42, by the attenuation filters 49 and 50 described above and output the attenuated light from the exit-side connector 43, the respective attenuation plates 49a, 49b, 50a, and 50b are rotated by drive sections 47 and 48, e.g., motors, to positions to obtain a desired attenuation amount.
However, in the optical attenuator 40 using the conventional attenuation filters 49 and 50 shown in FIG. 30, when the attenuation plates 49a and 50a are rotated to vary the attenuation amount, the optical path is blocked by the metal substrate of the attenuation plate 50a, and the attenuation amounts at the blocking portions become infinite, as shown by the attenuation characteristic of FIG. 31. This blocking occurs every time the attenuation plate is rotated to switch the attenuation amount.
In the optical attenuator 40 using the conventional attenuation filters 49 and 50 shown in FIG. 30, since the glass substrates are used in both the attenuation plates 49b and 50b, light blocking described above does not occur. However, a level variation occurs at transfer portions of the respective fixed attenuation amounts of the attenuation plate 50b, as shown by the attenuation characteristic shown in FIG. 32. Level variation differs depending on a difference in attenuation amount from the adjacent attenuation plate 49b and occurs inevitably at transfer portions.
In the optical attenuator 40 of this type, although not shown, the main shafts of the respective attenuation filters 49 and 50 are coupled to the main shafts of drive sections 47 and 48 each comprising a motor and an angular position detecting potentiometer which outputs a rotational angle of the corresponding attenuation filter as a voltage signal obtained from a change in resistance of a variable resistor.
When the drive sections 47 and 48 are operated to rotate the corresponding attenuation filters 49 and 50 by desired amounts and stop, the respective attenuation filters 49 and 50 are stopped by a rotational friction force of the drive sections 47 and 48 within a range of play of the drive sections 47 and 48, and a backlash always occurs. The backlash is caused mainly by the hysteresis of the potentiometer.
FIG. 33 shows a relationship between a preset angle (shift amount) and an attenuation amount obtained when the attenuation filter of the optical attenuator having a backlash caused by hysteresis of a potentiometer is continuously rotated in the forward direction, and a relationship between a preset angle and an attenuation amount obtained when the attenuation filter is continuously rotated in the reverse direction.
More specifically, an error of about, e.g., 0.05 to 0.2 dB occurs between the attenuation amounts obtained when the attenuation filter is rotated in the forward and reverse directions even when the preset angles are the same, as shown in FIG. 33. In other words, the influence of the backlash appears to exhibit the same attenuation amount even if the shift amounts of the potentiometer are different (D.sub.b and D.sub.a).
When the two attenuation filters 49 and 50 are controlled to obtain a desired attenuation amount, no problem occurs if the respective attenuation filters 49 and 50 are constantly rotated only in the forward or reverse direction. However, to obtain a desired attenuation amount within a short period of time by eliminating an unnecessary rotation, the attenuation filters 49 and 50 must be rotated in directions closer to target attenuation amounts from current positions, and the forward and reverse rotational directions of the attenuation filters 49 and 50 are controlled in accordance with the target attenuation amounts. Accordingly, even if the target attenuation amounts are the same, the actual attenuation amounts are different depending on the rotational directions of the attenuation filters, and desired attenuation amounts cannot be constantly obtained by the same value.
In each attenuator, when the attenuation amount is to be greatly varied, a plurality of infinite attenuation amounts or level variations are caused.
The problem of the variation in attenuation amount is that it adversely affects a system using an optical attenuator and, in particular, an error rate measuring system 150 including an optical repeater 51 shown in FIG. 34.
When the attenuation amount varies in an optical attenuator 52 during error rate measurement using the optical repeater 51, the light-receiving level of the measuring system varies instantaneously. This variation cannot be detected as an abnormal value or regarded as an error. In the worst case, measurement is disabled.
When the light-receiving level varies, as described above, a protection function is started to prevent the optical repeater 51 from being damaged by a spike current, and the measuring system is sometimes stopped.
Conventionally, in order to avoid malfunction of the protection function, a control program for estimating a level variation and its influence is separately prepared in advance as a control function of the entire system.